A variety of photographic imaging systems are known wherein a hardenable organic component containing ethylenic unsaturation sites is relied upon for image formation. The organic component undergoes photoinduced addition reactions, typically either polymerization or crosslinking, at the ethylenic unsaturation sites which produce hardening and allow image discrimination to be achieved.
It is common practice in preparing these compositions to employ coinitiators. One of the coinitiators is an activator. Following exposure the activator is relied upon to release a free radical which in turn promotes an addition reaction at the sites of ethylenic unsaturation. The other coinitiator is a photosensitizer which enhances photon capture and transfer of photon energy to the activator. An important function of the photosensitizer is to extend the wavelength range of useful exposures for hardening. By employing a dye as a photosensitizer the wavelength region of response is shifted from the ultraviolet to the visible spectrum. Thus, an important function of a photosensitizer is to shift the wavelength of response of the imaging system bathochromically.
Among the most efficient dyes for use as photosensitizer coinitiators are coumarin dyes. Specht et al U.S. Pat. No. 4,278,751 discloses a ketocoumarin coinitiator for free radical polymerization having a maximum absorption in the 350 to 550 nm range containing a 7-amino substituent to the coumarin ring. The 7-amino substituent can complete a julolidene ring fused with the coumarin ring.
Grychtol U.S. Pat. No. 3,932,446 discloses a 6-aminobenzofuran dye which is substituted in its 2 position with a cyano group, a carboxy acid or ester group, or a electron donating nucleus, such as a benzoxazole, benzothiazole, or benzimidazole. The 6-aminobenzofuran dyes of Grychtol are fluorescent dyes. Since the energy these dyes receive is dissipated in the form of fluorescence, they are not efficient photosensitizers.